(1) Field of the Invention
The instant invention relates to extendable fiber optic cables, and more particularly to a torque-balanced extendable fiber optic cable.
(2) Description of the Prior Art
It is well known in the art that fiber optic transmission cables are utilized in towed array sonar systems to transmit data from an underwater sonar sensor to a ship or other vehicle while the sensor is towed through the water by the ship or vehicle. It has been found that when the fiber optic cable is towed through the water, the cable is subjected to very high tensile forces which stretch the optical fibers located within the cable, and tend to degrade the quality of transmission through these optic fibers. In this regard, extendable or stretchable fiber optic cable designs have heretofore been known in the art. These extendable cables are operative for decreasing the tensile forces applied to the optical fibers in order to increase the quality of transmission.
The U.S. patents to Nowak U.S. Pat. No. 3,955,878; Jachimowicz U.S. Pat. No. 4,146,302; van der Hock U.S. Pat. No. 4,468,088; Holland et al U.S. Pat. No. 4,852,964; and Taketani et al U.S. Pat. No. 4,873,030 disclose cable structures which represent the closest prior art to the subject invention of which the applicant is aware. The patent to Nowak discloses a fiber optic transmission line having helically wound optical fibers which are positioned lengthwise within a flexible tubing. The fibers are separated from each other and suspended within the flexible tubing by means of a semi-liquid gel which lubricates and insulates the fibers from the cable structure.
The patent to Jachimowicz discloses a flexible fiber optic cable in which optic fibers are formed into an elongated strip and then the strip is coiled into a helix to allow bending and stretching motion without disruption of the optical transmission.
The patent to van der Hock discloses a flexible optical fiber transmission cable wherein a plurality of fibers extend therethrough in alternating left and right hand helixes.
The patent to Holland et al discloses a fiber optic cable assembly which is helically wound about a mandrel and heat set to retain its coil shape. The resulting coil shape provides a stretchable optical cable assembly.
The patent to Taketani et al discloses another method of molding a fiber optic transmission cable into a coil.
These prior art cable constructions are known to have several disadvantages. Firstly, the prior art cable constructions only allow a minimal degree of stretching before optical transmission is compromised. Further, although the prior art coiled cable structures are known to reduce axial or tensile stresses on the optical fibers, they also increase twisting or torque stresses to the optic fibers.